I have formulated basic theory on how unicorn magic works. The general idea goes, that it's the Tree of Harmony. When the Tree was dying, unicorn magic went berserk. Keep that in mind. Now, when a unicorn wants to cast a spell, they are sending a message to the Tree. Misfires happen when they sent a "typo". The Tree itself possesses large amounts of magical energy(lets just call it mana- the power of a place), while living things possess much smaller pools of magic(prana- the power of a person). Spellcasters trade in their prana for the Tree's mana- the Tree has to keep its stores up. Twilight and the alicorns are so powerful, because the Tree gives them a "discount", so to speak. A "cutie mark", is the Tree's way of showing that a mortal has a connection to it. Unicorns send the message down their connection, with a nice downpayment of prana, then the Tree sends mana, ready woven into spells to the unicorn's specifications, back through the link, and the unicorn channels it out, into the world through their horn, which acts as a natural focus for magical energy, mana or prana. The Elements(and later the Rainbow Power) are so powerful because they represent the Tree's trump card- the signal to fire everything at this guy. When Tirek stole everyone's magic, he was siphoning off their prana, and he happened to steal their connection along with it. When he casts a spell, the Tree thinks everyone who's magic he stole is focusing their power on this one spell. Now, prana regenerates naturally over time, but losing it tires a being out. When their connection to the Tree was stolen, their marks vanished. So, while the ponies would have recovered physically, they would no longer be able to cast spells as they once did. It's a sort of Dresden Filles mantles means A Divine Comity: Mysterious Ways white magic. When Tirek stole Discord's magic, he was only able to take his prana(raw energy), since Discord uses black magic- he weaves magic into spells himself, rather than white magic- calling on an outside force(such as the Tree, a circle of like-minded magicians, a deity, etc) for help. If Discord could have avoided being drained again, his prana would have recovered, and he'd be back in business. That said, I assume that the less prana you have, the longer it takes to recover it, and Discord had a LOT of prana. Anyway, I assume that's how magic works outside Equestria in general- Celestia specifically stated that Tirek came for "Equestrian Magic", which I assume means that he could only take the raw energy from beings who used black magic(he couldn't take their abilities for himself, only their prana, the raw energy). The Sirens, for instance, used the chaos they caused to tap into ponies connection to the Tree, and siphon some of its mana for themselves. When Twilight and the Humane 5 used the Element's power in Canterlot High, they formed a connection to the Tree within Canterlot High, so the Sirens could access mana again, instead of just siphoning prana of nearby humans. Why did they want mana, you ask? I have two ideas: A) they were specially adapted to be powered by mana rather than prana. One of the Sirens mentioned that siphoning from humans was like "fast food", and she wanted a meal and B) mana is just better/more plentiful. Which makes sense, why connect to the Tree if you have all the power you need inside of you?